A DCxe2x80x94DC converter developed under any topology, with or without isolation between input and output, is using a control signal to adjust the duty-cycle and regulate the output against input or load variations. The control section may comprise different configurations: direct duty-cycle voltage-mode control, feed-forward voltage-mode control, peak current-mode control or average current-mode control. By combining these multitude of topologies (buck, boost, flyback, etc) with different control options, a big variety of DCxe2x80x94DC conversion solutions can be achieved, to suit particular applications requirements (size, output power, power dissipation, output noise, input or output voltages). However, all existing topologies have a common problem, when dealing with transient events, like start-up, sudden variation of input voltage or load. During this relatively short period of time, the feedback control loop behavior is critical and will translate on how fast and accurate the power supply is adapting to the new conditions. There are physical limitations to an ideal and instant response from the feedback control circuit. Energy levels previously stored in the output inductors and capacitors, in the control loop compensation capacitors are impossibly to change as fast as the external conditions may vary. Consequently, there is a momentary discrepancy between the actual and needed control value, usually triggering dumped oscillations, resulting in unwanted control overshoots. This momentary open loop condition is wrongly generating an abnormal high ON time, with additional stress at the level of the power switches and magnetic components. FIG. 1 shows how different control configurations typically handle a transient event. The solution to this problem is to oversize the power switch, to handle the increased peak current and to oversize the magnetic components (number of turns and/or magnetic cross-section area) to prevent saturation because of higher flux density. This may not be acceptable in some designs, where the size is an issue. A method to overcome this problem is illustrated in FIG. 2. It consists in limiting the duty-cycle to a maximum by clamping the control signal to a fixed level. The disadvantage of this technique is that for wide input voltage variation is corresponding a high variation of the duty cycle, according to the following transfer functions:
Vo=VinD(RT/2L)xc2xdxe2x80x94For flyback topology (in discontinuous inductor current)
Vo=VinD/(1xe2x88x92D)xe2x80x94For flyback topology (in continuous inductor current)
Vo=VinDTOFFR/2Lxe2x80x94For buck topology (in discontinuous inductor current)
Vo=VinDxe2x80x94For buck topology (in continuous inductor current)
Vo=VinRDTOFF/2Lxe2x80x94For boost topology (in discontinuous inductor current)
Vo=Vin/(1xe2x88x92D)xe2x80x94For boost topology (in continuous inductor current)
where:
Vo=output voltage
Vin=input voltage
D=TON/T (duty cycle)
R=load resistance
T=switching period of time
TON=period of time when the switch is ON
TOFF=period of time when the switch is OFF
L=inductance value of the inductor
Generally emerging from the above transfer functions, for low input voltage corresponds high duty-cycle D (and control voltage) and vice-versa, if output voltage and current are constant. If fixed clamp is applied to control voltage (which determines duty-cycle D), for its maximum level (corresponding to low input voltage and full output power), this may not protect the magnetic cores from saturation if high input voltage and momentary overshoot of control voltage. Although this technique is limiting the overshoot of the feedback loop response, further improvements will be introduced by the invention presented below, conducting to further switches and magnetic components size optimization.
This invention offers reliable protection against over-current in the main switches and/or saturation of the magnetic components (power transformer and/or inductors) in a DCxe2x80x94DC converter built under any topology, by using a feed-forward clamping circuit to limit the feedback control signal over a wide range of input voltage. The result is an increase of reliability and enables optimization of the main switches and magnetic components (power transformer and/or main inductor) in the way that minimizes their overall size. The protection is active only during transient events, when momentary open loop condition may occur.